Despite recent substantial progress in diagnosis and therapy of breast cancer, most patients with advanced metastatic breast cancer still succumb to the disease. Therefore, there is an urgent need for developing novel anticancer agents to save lives of patients with advanced breast cancer. For the past several years we have been working to develop such novel anticancer agents. In this proposal, we will initiate clinical trials of one such novel anticancer agents, as well as perform laboratory studies to better understand the mechanisms by which the anticancer agent suppresses advanced established tumors in addition to inhibition of new tumors and metastasis. The novel anticancer agents are monoclonal antibodies (mAbs) defining novel antigenic determinants (epitopes) of an antigen termed endoglin. We initially reported Endoglin as a novel human leukemia-associated antigen. Later it was also found on endothelial cells as well as leukemia cells. Endoglin is essential for angiogenesis, which is a fundamental process for formation of new blood vessels. Tumor growth and metastasis are angiogenesis-dependent. Endoglin is strongly expressed on tumor-associated blood vessels. Unlike many known anti-angiogenesis agents, anti-endoglin mAbs showed vascular targeting activity in addition to anti-angiogenesis activity. In mouse model studies, anti-endoglin mAbs and their derivatives were able to induce regression of established tumors by vascular targeting activity, i.e., by destroying preformed blood vessels in established tumors. No overt toxicity was detected under the conditions of the therapeutic studies. We predict that these mAbs will be much more effective for destroying tumors in humans than in mice because these mAbs react with human blood vessels much more strongly (approximately 20 times) than with murine blood vessels. In developing a novel anticancer agent, we need to carefully evaluate potential toxicity/side effects in addition to antitumor efficacy. The potential toxicity of an anti-endoglin mAb was evaluated in monkeys and mice and no overt toxicity was detected in either animal species. To facilitate clinical application of a selected anti-endoglin mAb, the major part of the mAb was humanized by a recombinant gene technology. The humanized mAb maintained the same antigen-binding activity and specificity as the parental murine mAb. The mAb showed no overt toxicity in monkeys in dose-escalation tests. This mAb will be used for clinical trials in breast cancer patients at our institute where many breast cancer patients are seen and treated. We hope that results of the clinical trials will provide us with evidence that this novel anticancer agent is indeed effective for treating patients with advanced metastatic breast cancer. It is expected that our novel anticancer agent can be applied to the therapy of most breast cancer patients regardless of the status of HER-2 expression.